Related Field
The present invention relates to method for estimating values for a set of parameters of an imaging system and to an apparatus arranged to take pictures with an imaging system using a method for estimating values for a set of parameters of the imaging system.
Description of Related Art
Optical imaging systems in practice never give a perfect image of a scene which is photographed. This is due to a lot of physical effects like aberration and well described in books about optics, like “Principles of Optics” by Max Born and Emil Wolf. However, there exist a lot of different techniques to minimise aberrations and to correct, at least partially, some of them. These minimisations and corrections can be done in the imaging systems itself, for example, by careful design of them. If the imaging system gives pictures which are accessible electronically, for example, via scanning them or via taking them directly via a CCD (charge-coupled device)-camera or a camera using a CMOS (complementary metal-oxide-semiconductor)-sensor, a correction could also be done with the help of computer programs.
Often, it is desirable to get information from a picture about the scene or objects which are on a picture. This could, for example, be size, shape, colour or other properties of the scene or the objects. For being able to get this information one needs to know some properties of the optical system, for example, one or more of the following properties: focal length(s), field of view, thicknesses and positions of the lenses, indexes of refraction, distortions. These properties are either known, or could be measured to calibrate the imaging system. The calibration can, for example, be done by test pictures with known properties, or by photographing other known objects. In these calibration procedures one usually uses a mathematical/physical model to conclude from the properties of a picture to the properties of the imaging system.
However, the pre-known values of the parameters of the imaging system or the values of the parameters of the imaging system obtained from the calibration procedure can change by time. Mechanical stress might change the relative position of the different components in the imaging system. This might, for example, result in that components get closer or further away, that components might turn relative to each other or that components might be moved closer to or further away from the optical axis. A change of temperature might also change the relative position of the components, for example, if the components are mounted on a common rail or on a common tube (or any other common holding element) and that holding element expands or contracts due to a change in temperature. Also the index of refraction of some components or the performance of some components, like CCD-chips, can change by a change in temperature. Even other influences from the environment can change the components of the imaging system and/or the properties of them, resulting in an imaging system, which behaves differently than when it was calibrated. When one tries to make conclusions from the pictures of such a differently behaving system about the scene or the object(s) which have been photographed, one might end up at wrong information about the scene or the object(s).
Prior art solutions to solve that problem try to minimise any possible changes by careful design of the imaging system or by real-time monitoring different parameters of the imaging system. However, careful design can make the systems complex and heavy and might only minimise the above mentioned changes in the imaging system, but never completely eliminate them. Also real-time monitoring can make the systems complex and heavy and it might be hard to monitor all possible changes in the system.
Especially when the imaging system is mounted on airplanes high temperature changes and high mechanical stress might occur, resulting to change the behaviour of the imaging system in such a way that careful design does not protect against all possible effects from the environment and that monitoring of all the parameters needed is not feasible due to space constraints and due to fact that the monitoring systems of all important parameters might be affected by the same stress and therefore might malfunction due to the tough environment.